1) Field of the Invention
The present invention relates to a process for the preparation of a high-permittivity material, and more specifically to a process for the preparation of a high-permittivity material suitable for use as, for example, a microwave dielectric resonator or a substrate for a microwave integrated circuit.
2) Description of the Related Art
With increase of information contents through communications, communication means through microwave are rapidly advanced in fields of mobile telephone systems, satellite communications, satellite broadcasts and the like. Parts used in these communication means are required to enhance their performance, make their dimensions small and reduce their prices.
In this case, regarding microwave dielectric resonators making use of a ceramic material by way of example, the wavelength of the electromagnetic wave propagated through the interior thereof is shortened in proportion to 1/.sqroot..epsilon..sub.r (.epsilon..sub.r : relative dielectric constant) compared with that through air. Therefore, such resonators can be made in a small size compared with cavity resonators and hence have been extensively used in recent years. Besides, the ceramic material used therein has also been used as a substrate for microwave integrated circuits owing to its high relative dielectric constant and low dielectric loss.
A material for a microwave dielectric resonator must meet requirements such as a high dielectric constant, low dielectric loss and a low temperature coefficient of resonant frequency. As materials satisfying such requirements, there are known those of the BaO-TiO.sub.2 type, SnO.sub.2 -ZrO.sub.2 -TiO.sub.2 type, BaO-TiO.sub.2 -Nd.sub.2 O.sub.3 type and complex perovskite type typified by Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3.
In general, dielectrics making separate use of the above-mentioned materials are sintered bodies obtained by a firing process under atmospheric pressure and hence have voids in their interiors in proportions of several % in terms of volume. In this case, the proportion of the voids contained inside the sintered body (void content) is affected by the firing temperature, the particle size of raw materials for the ceramic, impurities, etc. Therefore, the void content often varies depending upon the production lots and firing lots of a raw material. Even in the same lot, the values of the void content tend to vary depending upon the firing positions of the raw material because of temperature and atmosphere distributions in a kiln. Accordingly, it is difficult to obtain a dielectric having stable properties.
In the dielectric resonator on the other hand, its resonance frequency is determined by the dielectric constant of its material, and dimensions and configuration characteristic of its resonator. In this case, it is desirable that the dispersion in resonant frequency between dielectric resonators should be as small as possible. However, since there are variations of void content in the dielectric obtained in the above-described manner and the relative dielectric constant (about 1) of air present in the voids is very low compared with that (10-100) of the ceramic material, great dispersion in dielectric constant occurs and hence the resonance frequency is also dispersed widely, so that the yield of the dielectric resonators is reduced to a significant extent. In a microwave band, high accuracy for a desired resonance frequency is often required, in particular, of the SHF band (3 GHz or higher higher in frequency). There is hence a great problem therein. Therefore, in order to avoid such disadvantages, additional processes, for example, a process in which the dimensions of individual dielectrics are changed to fabricate resonators having the same resonance frequency, are required.
In the case of the substrate for a microwave integrated circuit on the other hand, its thickness must be made even in order to make the dispersion of characteristic impedance as to wiring smaller. In addition, since the wiring is composed of a thin film formed by sputtering, vapor deposition, plating or the like, the substrate must be polished prior to its use. In this case, when the voids present inside the substrate appear on the surface thereof to turn into pores, any minute patterns cannot be formed on the surface. In particular, when a material high in dielectric constant is used, a minute pattern must be formed due to the problem of characteristic impedance. Therefore, the presence of voids become a great problem.
As means for reducing the void content of a ceramic material, are known hot pressing and hot isostatic pressing (HIP). However, these methods are accompanied by great problems because ceramic materials for microwave dielectric resonators, which have been known to date, are all oxides.
Namely, with respect to the hot pressing, the firing temperature can be raised only to about 1,300.degree. C. from the viewpoint of heat resistance of a jig. Therefore, materials allowed to fire at such a temperature are limited, and moreover such a method is not suitable for mass production, so that production cost becomes expensive. With respect to HIP on the other hand, a relatively large amount of the material can be treated. However, such a treatment is accompanied by a problem that since the treatment is principally conducted in a non-oxidizing atmosphere such as N.sub.2 or Ar, ceramic dielectrics which are oxides are reduced when applying such a treatment thereto, so that the dielectric loss of the dielectrics is remarkably deteriorated.